1. Field of the Invention
The present invention relates to a subtractor circuit and an operational amplifier, and particularly to a subtractor circuit and an operational amplifier that are advantageous for driving at a low voltage.
2. Description of the Related Art
An IC (Integrated Circuit) used for control of an electronic device or the like generally includes electronic parts such as an operational amplifier and the like. The operational amplifier includes semiconductor elements such as BJTs (Bipolar Junction Transistors), MOSs (Metal Oxide Semiconductors) or the like. The circuit configuration of the operational amplifier is substantially the same regardless of whether the semiconductor elements forming the operational amplifier are BJTs or MOSs.
FIG. 1 is a circuit diagram showing the configuration of an example of an existing operational amplifier including MOSs (see Japanese Patent Laid-Open No. Hei 4-185005 referred to as Patent Document 1 hereinafter, for example).
In FIG. 1, an operational amplifier 11 includes a subtractor circuit 12, an output amplifier circuit 13, an inverting input terminal 14, a non-inverting input terminal 15, and an output terminal 16.
The subtractor circuit 12 includes PMOSs (Positive Metal Oxide Semiconductors) 21 and 22, NMOSs (Negative Metal Oxide Semiconductors) 23 and 24, and a constant-current source 25.
In the subtractor circuit 12, the gate of the PMOS 21 is connected to the inverting input terminal 14, and the gate of the PMOS 22 is connected to the non-inverting input terminal 15. The source of the PMOS 21 and the source of the PMOS 22 are connected to one terminal of the constant-current source 25. Another terminal of the constant-current source 25 is connected to a power supply not shown in the figure for supplying a driving voltage E1.
Further, in the subtractor circuit 12, the drain of the PMOS 21 and the drain of the NMOS 23 are connected to each other, and the drain of the PMOS 22 and the drain of the NMOS 24 are connected to each other. The sources of the NMOSs 23 and 24 are each grounded. The gate of the NMOS 23 and the gate of the NMOS 24 are connected to each other. A point of connection between the gate of the NMOS 23 and the gate of the NMOS 24 is connected to a point of connection between the drain of the PMOS 21 and the drain of the NMOS 23. A point of connection between the drain of the PMOS 22 and the drain of the NMOS 24 is connected to the output amplifier circuit 13.
The output amplifier circuit 13 includes an NMOS 31, a capacitor 32, and a constant-current source 33.
In the output amplifier circuit 13, the gate of the NMOS 31 and one terminal of the capacitor 32 are connected to each other. A point of connection between the gate of the NMOS 31 and the one terminal of the capacitor 32 is connected to the point of connection between the drain of the PMOS 22 and the drain of the NMOS 24 in the subtractor circuit 12. The source of the NMOS 31 is grounded. The drain of the NMOS 31 is connected to another terminal of the capacitor 32, one terminal of the constant-current source 33, and the output terminal 16. Another terminal of the constant-current source 33 is connected to a power supply not shown in the figure for supplying the driving voltage E1.
In the operational amplifier 11, a first input voltage is input to the inverting input terminal 14, and a second input voltage is input to the non-inverting input terminal 15. A voltage obtained by subtracting the first input voltage from the second input voltage is supplied from the subtractor circuit 12 to the output amplifier circuit 13. The output amplifier circuit 13 then amplifies the voltage supplied from the subtractor circuit 12. The voltage amplified by the output amplifier circuit 13 is output as an output voltage from the output terminal 16.
The thus formed operational amplifier 11 is generally referred to as a Barton amplifier, and is generally used in a bipolar process and a MOS process.
In this case, the constant-current source 25 is also formed by a semiconductor element, and the subtractor circuit 12 in the operational amplifier 11 is formed by stacking semiconductor elements in three stages. That is, as shown in FIG. 1, the subtractor circuit 12 has a configuration such that the constant-current source 25, the PMOS 21 or 22, and the NMOS 23 or 24 are connected in series with each other between the driving voltage E1 supplied to the constant-current source 25 and a ground level.
Hence, a voltage supplied to each of the constant-current source 25, the PMOS 21 or 22, and the NMOS 23 or 24 is lower than the driving voltage E1. Thus, in order to drive each of the constant-current source 25, the PMOS 21 or 22, and the NMOS 23 or 24, a voltage equal to or higher than a voltage obtained by adding together voltages necessary to drive the constant-current source 25, the PMOS 21 or 22, and the NMOS 23 or 24, respectively, needs to be set as the driving voltage E1.
Thus, because a voltage equal to or higher than the voltage obtained by adding together the voltages necessary to drive the constant-current source 25, the PMOS 21 or 22, and the NMOS 23 or 24, respectively, needs to be set as the driving voltage E1, the subtractor circuit 12 and, in turn, the operational amplifier 11 may not be suitable for driving at a low voltage.